A molecular spectrometer (sometimes referred to as a spectroscope) is an instrument wherein a solid, liquid, or gaseous specimen is illuminated, often with non-visible light, such as light in the infrared region of the spectrum. The light from the specimen is then captured and analyzed to reveal information about the characteristics of the specimen. As an example, a specimen may be illuminated with infrared light having known intensity across a range of wavelengths, and the light transmitted and/or reflected by the specimen can then be captured for comparison to the illuminating light. Review of the captured spectra (i.e., light intensity vs. wavelength data) can then illustrate the wavelengths at which the illuminating light was absorbed by the specimen, which in turn can yield information about the chemical bonds present in the specimen, and thus its composition and other characteristics. To illustrate, libraries of spectra obtained from reference specimens of known composition are available, and by matching measured spectra versus these reference spectra, one can then determine the composition of the specimens from which the measured spectra were obtained.
Two common types of spectrometers are dispersive spectrometers and Fourier Transform (FT) spectrometers. In a dispersive spectrometer, a range of input light wavelengths are supplied to a specimen, and the output light from the specimen is received by a monochromator—a device which breaks the output light into its component wavelengths—with one or more detectors then measuring light intensity at these output wavelengths to generate the output spectrum. In an FT spectrometer, an interferometer is used to supply an interferogram—a time-varying mixture of several input light wavelengths—to a specimen, and one or more detectors measure the (time-varying) output light from the specimen. The various wavelengths of the output light can then be “unscrambled” using mathematical techniques, such as the Fourier Transform, to obtain the intensity of the output light at its component wavelengths and thereby generate the output spectrum.
Spectroscopic microscopes then usefully incorporate the ability to make spectroscopic measurements into an optical microscope. A user may therefore use a spectroscopic microscope to view an image of a region of interest on a specimen (usually in magnified form), and also to obtain spectroscopic data from one or more locations on the region of interest. The spectroscopic measurements are often obtained by capturing spectroscopic data along a 1-dimensional row of areas on the region of interest (i.e., at areas spaced along a line on the region of interest), and then repeatedly capturing spectroscopic data from adjacent 1-dimensional rows. In other words, the linear array of spectroscopically-sampled areas is stepped sideways to ultimately capture spectroscopic data over a 2-dimensional array of areas over the region of interest. As a result, the user can view an image of the region of interest, and can also review the spectra (and thus the composition) of the specimen over the region of interest.
An important issue for users is the spatial resolution of a spectroscopic microscope, i.e., the size of the area over which each spectroscopic measurement is taken. Since each spectroscopic measurement from an area effectively provides the spectra of all substances present in the area (or at least the spectra of those substances which are responsive to the incident light), a spectroscopic measurement over a large area can be “coarse”: it can reflect the presence of numerous substances, though a user may not be able to visually assign each detected substance to a particular area seen through the microscope. In other words, while a user might see an area and spectroscopically determine the presence of certain substances, a user might not know where each substance specifically resides on the area—and such information can be highly useful. In contrast, a spectroscopic measurement taken over a smaller area can provide the desired information, since the location of the detected substance(s) is more specifically located. However, spectroscopic measurements captured over a smaller area tend to have a lower signal-to-noise ratio (i.e., they are noisier). This is largely because less light can practically be detected from a smaller area, and the lesser detected light leads to lower signal levels. A user can compensate for a lower signal-to-noise ratio by increasing the exposure time (i.e., the time over which the area is illuminated, and over which light is detected therefrom), and/or by taking multiple exposures from the area and combining them (e.g., averaging them), which tends to attenuate the effect of noise. These procedures require greater measurement time, which is also undesirable from the user's standpoint.
Because of the foregoing tradeoff between spatial resolution and signal-to-noise ratio, it is useful for users to have variable resolution in a spectroscopic microscope—in effect, the ability to shift between coarse resolution wherein spectral measurements are taken over larger areas, and fine resolution wherein spectral measurements are taken over smaller areas. An example of such an arrangement is illustrated in published U.S. Patent Appln. 2002/0034000 to Hoult et al., and in its associated U.S. Patent Appln. 2002/0033452 to Hoult et al. In these references, variable resolution is provided by inserting a set of magnifying optics within the light path between the specimen and the spectroscopic detector. The problem with this approach is that it introduces yet another drawback: “vignetting,” a condition wherein the light from the area(s) under analysis on the specimen is non-uniformly supplied to the spectroscopic detector. If one considers that the detector effectively receives a projected optical image of each area under analysis, vignetting usually involves the problem that the center of the projected image of the area is bright—the integrity of the image is preserved—but the brightness decreases toward the edges of the image. Vignetting therefore leads to the problem that the spectrometric measurements from the specimen area are skewed, with the nonuniform illumination across the received image providing higher signal levels nearer the center of the image/area, and diminished signal levels near the edges of the image/area. As a result, the spectra from the substance(s) at the center of the area/image are more strongly represented in the area's spectroscopic measurements than the substance(s) nearer the edges of the image/area. More generally, vignetting leads to less than optimal signal strength owing to the loss in light near the edges of the image/area. Vignetting tends to arise as a side-effect from the use of “off-angle” arrangements of optical elements, i.e., from reflection or refraction of light at angles to the optical axis of the lenses and/or mirrors used for the microscope optics (at least those lenses/mirrors used to perform spectroscopic measurement functions). The use of greater off-axis angles tend to lead to greater vignetting, and thus greater reductions in image luminance and signal strength, because light is lost as it “leaks” from the usable areas (the apertures) of successive optical elements. In a spectroscopic microscope, it is often possible to adapt the optics so that vignetting is minimized for one of the resolution/magnification settings, but this then tends to enhance vignetting in the other settings as additional optics are introduced to enable the settings. It would therefore be useful to have a means for reducing the effects of vignetting in spectrometers and spectroscopic microscopes, and to allow the use of variable spatial resolution settings with reduced vignetting effects.